The present invention relates generally to emission control systems for electronically controlled compression ignition engines, and more particularly to an emissions control system having an emissions diagnostic system.
Compression ignition engines provide advantages in fuel economy, but produce and emit both NOx (nitrogen oxides) and particulates during normal operation. When primary measures (actions that affect the combustion process itself, e.g., exhaust gas recirculation and engine timing adjustments) are taken to reduce one, the other is usually increased. Thus, combustion conditions selected to reduce pollution from particulates and obtain good fuel economy tend to increase the output of NOx. Current and proposed regulations and legislation challenge manufacturers to achieve good fuel economy while at the same time require the reduction of the emissions of particulates and NOx.
In order to meet such requirements or restrictions a method known as SCR (selective catalytic reduction) has been used for reducing the emission of NOx. The SCR method consists of injecting gaseous ammonia NH3, ammonia in aqueous solution or urea, into the exhaust gas system of the compression ignition engine as a reduction agent. Originally, SCR depended on the use of ammonia, which has safety problems associated with its storage and transport. Aqueous urea and solid reagents are safer, but were not initially practical for many SCR applications, particularly mobile NOx sources, due to the difficulty in converting the aqueous urea or the solid reagents from solution or solid form to active gaseous species, typically NH3 and HNCO radicals.
Because of the heightened awareness and concern with emissions, there was a need for a safe, economical and effective answer to the problems associated with SCR, particularly for mobile compression ignition engines such as diesel engines. Where SCR catalysts had been employed to limit NOx emissions from compression ignition engines, one had to deal with either the dangers of ammonia leakage or using a urea solution or other reagent and risk fouling the catalysts under most conditions. The first limited attempts to use urea for compression ignition engines sometimes required the use of large pyrolization chambers or other devices following the point of urea introduction into the exhaust from the engine. Although there were initial problems in using urea, such problems have been for the most part solved.
However, such prior art urea based emissions control systems generally have no diagnostic system to insure that the system is operating properly and that the system has not been tampered with in an attempt to override the emissions system. One such prior art emissions control system has the capability of detecting the level of urea in the system""s urea storage tank. When the storage tank is empty the system will retard timing of the engine to thereby decrease the NOx emissions to an acceptable level. Since fuel consumption is increased as the timing is retarded, the operator will have an incentive to refill the urea storage tank as quickly as possible in order to return the engine to a higher fuel economy level. Although such a prior art system is useful, it does not eliminate the possibility of misuse or tampering. For example, since urea is relatively expensive, the operator may attempt to defeat the system by simply filling the urea tank with water. In such an event the operator would achieve the desired fuel economy without having to purchase urea, but at the expense of increased emissions.
While the prior art systems generally perform acceptably while there is NOx reducing reagent present in the storage tank, the prior art systems do effectively handle the situations where the storage tank is empty of urea or when the storage tank is filled with a substance other than urea. It would be preferable to have a diagnostic system that could insure that the emissions system is operating properly and the system has not been purposely defeated or tampered.
The present invention relates to an emissions diagnostic system for a compression ignition engine having an exhaust gas stream which is directed into an SCR catalyst capable of reducing NOx in the exhaust gas stream. The system comprises a tank for storing a solution of urea, a pump in fluid communication with the tank for drawing the solution of urea from the tank, a valve in fluid communication with the pump and the exhaust gas stream for receiving the solution of urea from the pump, an air pump connected to the valve for providing air into the valve for mixing with the solution of urea, the valve for spraying the solution of urea into the exhaust gas stream, a sensor for sensing the level of NOx emissions emitted by the SCR catalyst, and a control module connected to the sensor for determining whether the level of NOx emissions is at an acceptable level.
In another aspect of the invention relates to an emissions diagnostic system for reducing NOx emissions from an exhaust gas stream of a compression ignition engine with the exhaust gas stream being passed through an SCR catalyst effective for selective catalytic NOx reduction. The system comprises a tank for storing a solution of urea, the tank further having a heat source to apply heat to the solution of urea, a pump connected to the tank for pumping the solution of urea from the tank, a valve connected to the pump and exhaust gas stream, the valve for receiving the solution of urea from the pump, an air pump connected to the valve for providing air into the valve for mixing with the solution of urea, the valve for controllably spraying the solution of urea into the exhaust gas stream, a sensor associated with an output of the SCR catalyst for sensing the level of NOx emissions emitted by the SCR catalyst, and a controller connected to the sensor for determining whether the level of NOx emissions is within a predetermined range.
Other aspects and advantages of the present invention will be apparent to those skilled in the art upon reading the following detailed description in connection with the drawings and appended claims.